Mechanical seals are utilized for sealing a space between a rotary shaft and a casing of a machine to which the rotary shaft is adapted. Mechanical seals include a rotary ring mounted on the rotary shaft with a packing and a non-rotary ring mounted on the casing with a packing. The rings form sealing faces which seal said space. One of the rings is slidable in the axial direction of the rotary shaft and it is forced to be slidably in contact with another ring at the sealing face by springs and pressure of a medium to be sealed. The slidable ring compensates wear of the sealing faces or follows the vibration of the shaft or the like.
Mechanical seals are more reliable than other sealing device, for example gland packings. However even with mechanical seals, leakage of contents to be sealed sometimes happens. The medium is leaked from the sealing face or from the gap between the rotary ring and the rotary shaft due to the poor sealing thereof. The sealing faces sometimes happen to be separated each other due to the vibration of the rotary shaft or the like and due to wear of the faces. As described above, one of the sealing ring is mounted so slidable as to compensate the wear and to follow the movement in the axial direction of the rotary shaft. However the slidable ring can follow only some extent of vibration of the rotary shaft. In an agitator or the like where a longer rotary shaft is equiped thereon, the mechanical seal mounted on the shaft often causes the leakage due to the vibration of the longer rotary shaft. Furthermore the slidable ring have a tendency to become adhered on the rotary shaft which prevents slide movement of the ring. Such adhesion of the slidable ring is caused by many reasons, for example invasion of slurry into the gap between the slidable ring and the rotary shaft, bite of the packing into the gap between the slidable ring and the rotary shaft and adhesion of the packing or the like. The ring which become non-slidable causes separation of the sealing faces and leakage of medium.